1. Field of the Invention
The present invention relates to the field of gas turbine engines and is intended more particularly to make it easier to manufacture and mount a low-pressure turbine in a gas turbine engine.
2. Description of the Related Art
A front-fan and bypass turbojet, for example, conventionally comprises, from upstream to downstream, a fan, a low-pressure compressor stage, a high-pressure compressor stage, a combustion chamber, a high-pressure turbine stage and a low-pressure turbine stage.
By convention, in the present application, the terms “upstream” and “downstream” are defined relative to the direction of travel of the air in the turbojet. Similarly, by convention in the present application, the terms “inner” and “outer” are defined radially relative to the axis of the engine. Therefore, a cylinder extending along the axis of the engine comprises an inner face turned toward the axis of the engine and an outer face opposite to its inner face.
A low-pressure turbine comprises successive rotor disks each comprising axial or oblique grooves in which blade roots are engaged, the blades extending radially outward relative to the axis of the engine. The blade roots are held radially in the grooves of the rotor disk by their bulbous section, called dovetailed and, axially, by an upstream annular retaining ring in axial abutment on an upstream portion of the blade roots. The grooves of the rotor disk are conventionally made by a method known to those skilled in the art as “broaching” which consists in drilling longitudinally, with the aid of a substantially conically shaped bit, the radially outer portion of a plain that is to say an ungrooved, rotor disk. When the grooves are formed, the bit passes right through the rotor disk.
One of the challenges that the aviation industry has to overcome consists in reducing the weight of the engines. To achieve this objective, it has been proposed to combine the consecutive rotor disks into a single part, called a “spool”. A rotor spool takes the form of an axial cylinder in which one or more series of blades are mounted. Accordingly, when the rotor spool comprises two series of blades, which is the equivalent of two rotor disks, it is conventionally called “Spool 1-2”. A rotor spool, in comparison with a plurality of rotor disks, makes it possible to dispense with inter-disk connecting means and thus to lighten the engine. Such a spool is shown in patent application FR 0958567, not published, by SNECMA.
With reference to FIG. 1, a rotor spool 10 according to the prior art comprises an upstream portion 10A in which are arranged upstream grooves 14A in order to retain a plurality of upstream blades 4A and a downstream portion 10B in which are arranged downstream grooves 14B in order to retain a plurality of downstream blades 4B. The downstream portion 10B is connected to its upstream portion 10A by a frustoconical portion 11 that is flared from upstream to downstream. In other words, the downstream portion 10B is offset radially outward relative to the upstream portion 10A of the spool 10.
For such a spool 10, the broaching of the grooves 14B of the downstream portion 10B of the spool 10 can be applied in a conventional manner, the bit passing right through the downstream portion 10B of the spool 10 in order to form the downstream grooves 14B of the downstream blades 4B. The bit moves from downstream to upstream in the downstream portion 10B of the spool 10.
On the other hand, the broaching of the upstream grooves 14A of the upstream portion 10A of the spool 10 cannot be carried out. Specifically, the bit cannot move from downstream to upstream in the upstream portion 10A of the spool 10 because of the downstream portion 10B of the spool 10 which prevents it being positioned downstream. Moreover, the bit cannot move from upstream to downstream in the upstream portion 10A of the spool 10 because the bit would complete its travel in the frustoconical portion 11 of the spool 10 which would damage it. In other words, the bit cannot pass right through the upstream portion 10A of the spool 10 which is a drawback.
An immediate solution would then be to have a rotor drum in two portions (an upstream portion and a downstream portion) that could then be broached independently of one another. It is then sufficient to weld the two portions together. However, this means that the rotor spool must be heat treated after welding. Such a treatment is likely to deform the spool, which is undesirable.
Another immediate solution, with reference to FIG. 1, consists in connecting the upstream and downstream portions by friction welding. However, such a welding induces inaccuracies of axial positioning of the upstream portion of the spool with its downstream portion, which is a drawback.
In addition to this drawback, a rotor spool, as shown in FIG. 1, is difficult to install in the engine. For an installation from the rear of the engine, the elements of the engine are inserted from downstream to upstream. For a one-piece spool, if the spool 10 is installed with its upstream blades 4A and its downstream blades 4B, it is difficult to place the stator blades 5 situated between the two series of blades because the downstream portion 10B of the rotor spool 10 prevents access to the location of installation.